Micro-system components have been developed over the last few decades on the basis of the technologies implemented for the production of electronic circuits.
They generally comprise a metallic beam or a membrane of small thickness, kept suspended by supports above mutually insulated conducting surfaces. A control electrode placed under the conducting surfaces and optionally separated from the said conducting surfaces by an insulating layer completes the device.
The membrane/control electrode assembly is subjected to an electric voltage by means of the control electrode. In the absence of applied voltage, the membrane is suspended above the conducting surfaces and there is no electrical contact between the said surfaces.
In general, radiofrequency or microwave-frequency MEMS micro-switches are not used as simple breakers. Indeed, direct contact between the membrane and the conducting surfaces or the control electrode appreciably decreases the lifetime of the device. A dielectric layer is interposed between the surfaces and the membrane. The simple function is thus transformed into a variation in the capacitance of a capacitor whose plates consist on the one hand of the membrane and on the other hand of the control electrode opposite. The capacitance then varies from a value Cup to a value Cdown.
The main advantages of this type of device are essentially:                production techniques which are derived from conventional technologies for fabricating electronic integrated circuits. They make it possible to simplify production and integration and consequently, to obtain low fabrication costs as compared with those of other technologies, while guaranteeing high reliability;        very low electrical powers consumed, a few microwatts being required for activation;        size. A micro-switch is thus produced within an area of the order of a tenth of a square millimeter, making it possible to achieve high integration capacity;        microwave-frequency performance. This type of micro-switch exhibits very small insertion losses, of the order of a tenth of a deciBel, much lower than those of devices affording the same functions.        
In general, the deformable upper membrane is produced by deposition of one or more layers of materials, at least one of these layers being a conducting material. These materials are those customarily used in micro-electronics.
A particularly beneficial application of these microsystems resides in their use as microwave-frequency switches. The manner of operation of this type of switch is notably illustrated in FIGS. 1, 2 and 3.
In the initial position, the membrane 11 is situated at a distance d with respect to an RF line 12, on which a nitride layer 13 is deposited as illustrated in FIG. 1. Assuming that the RF line is also used as electrode, the two ends of the membrane are earthed 14 as illustrated in FIG. 2.
If a potential difference V is applied between the electrode and the membrane, the two parts are brought closer together by attracting the membrane towards the lower electrode (the RF track).
At a value V of the voltage, the displacement of the membrane exceeds a third of the initial gap. Thus the membrane collapses onto the lower electrode as illustrated in FIG. 3. The switch is said to be in the down position and this voltage value is dubbed the activation voltage.
When the membrane is in the up position, illustrated in FIG. 1, the RF signal passes along the RF line without being disturbed.
When the membrane is in the down position the signal passes along the RF line and is short-circuited by the membrane, thereby creating a reflection of the EM wave (microwave-frequency signal) on the membrane, the signal does not cross the RF MEMS switch.
The actuation used for the RF MEMS switch of FIG. 3 is an electrostatic actuation performed by applying a potential between the line (bottom electrode) and the membrane (top electrode). Other actuations are conceivable such as thermal, piezoelectric, magnetostatic or hybrid actuations (using two or more of the aforementioned four actuations).
The type of contact between the membrane and the line is of capacitive type on the RF MEMS switch of FIG. 3, that is to say a dielectric layer has been deposited on the bottom electrode. The line, the dielectric layer, the air gap and the membrane form a variable capacitance making it possible to allow through or to block the microwave-frequency signal. The second possible type of contact is ohmic contact (metal-metal) between the membrane and the line.
The central line is overlaid with a dielectric at the level of the membrane to prevent there being any ohmic contact and therefore a flow of charge when the membrane is in the down state. This gives the advantage of zero, or almost zero, consumption of power to keep the membrane in the down state by making use of the central line as actuation electrode.
This use is nonetheless not without consequence on the useful lifetime of the dielectric which gradually becomes electrically charged through use and actuation.
Indeed, when the membrane attains the down state, a conventional capacitive charge effect occurs in the dielectric between the line and the membrane, causing charge trapping in the dielectric (positive if the electrons are torn from the dielectric, negative if the electrons are imprisoned in the dielectric).
The performance of the switch is impaired as the dielectric becomes charged. The final and irreversible effect of this is to lead to a membrane remaining stuck by electrostatic force to the dielectric, definitively locking the RF MEMS Switch in the down state, thereby signifying the “death” of this RF MEMS switch.